This invention relates to a current amplifier, comprising an input terminal for receiving an input current, a first and a second output terminal for supplying a first and a second output current, respectively, a first transistor whose collector-emitter path is arranged between the first output terminal and the input terminal and whose base is coupled to a bias voltage terminal for receiving a bias voltage. A first current mirror includes an input terminal, an output terminal and a common terminal coupled to the input terminal of the current amplifier, to the second output terminal of the current amplifier, and to a power-supply terminal, respectively. The input terminal of the first current mirror is coupled to the common terminal by at least two series-connected semiconductor junctions.
Such a current amplifier is known from European Patent Specification EP 0,055,724, FIG. 2. In this known current amplifier the input current applied to the input terminal of the current amplifier is branched into two parts. The first part flows to the first output terminal via the emitter-collector path of the first transistor. The second part flows to the input terminal of the first current mirror. The output of the first current mirror is coupled to the second output terminal of the current amplifier. A suitable load is connected to the output terminals in order to obtain the first and the second output currents of the current amplifier. When the input current is zero the magnitude of the quiescent currents flowing in the first and the second output terminal are determined by the bias voltage on the bias-voltage terminal, which is coupled to the base of the first transistor, and by the current transfer of the first current mirror. The emitter line of the first transistor and the series-connected semiconductor junctions of the first current mirror comprise resistors, but these resistors limit the swing of the first and second output currents. When an increasing positive input current is applied to the input terminal of the current amplifier the voltage at this input terminal will increase as a result of an increasing voltage drop across the resistor in the series-connected semiconductor junctions. Since the bias voltage on the base of the first transistor is constant the voltage increase on the input terminal of the current amplifier will cause the current through the first transistor to decrease. For a specific value of the input current the current through the first transistor, and hence the first output current of the current amplifier, will become zero. Conversely, in the case of an increasing negative input current the voltage on the input terminal of the current amplifier will decrease as a result of an increasing voltage drop across the resistor in the emitter line of the first transistor. Eventually the voltage decrease at the input terminal of the current amplifier will cause the series-connected semiconductor junctions of the first current mirror to be turned off, so that the second output current of the current amplifier becomes zero. The swing of the first and the second output current is increased by reducing the values of said resistors or by short-circuiting these resistors. When the resistors are fully short-circuited the output swing is maximal but the portions of the input current which are branched to the emitter of the first transistor and to the input of the first current mirror are now unequal. As a result of this, the relative current variations in the first output current of the current amplifier in response to an input current variation will be substantially larger than the relative current variations in the second output current of the current amplifier. In the case of an adequate drive the minimum value of the first output current may decrease so far that the bandwidth of the first output current decreases because the transition frequency of the first transistor decreases for small currents. The signal distortion in the first output current as a result of the decreasing bandwidth limits the effective swing of the first output current and consequently limits the output swing of the entire current amplifier.